The future LTE-A (Long Term Evolution-Advanced) system supports a transmission bandwidth up to 100 MHz, while in the current LTE standard the maximum supportable transmission bandwidth is 20 MHz such that a plurality of carriers need to be aggregated for broader bandwidth transmission. Carrier aggregation is a technique presented by 3GPP to support the higher requirement for transmission bandwidth in the future mobile communication system, in which a plurality of carriers are aggregated for transmission. The introduction of the CA technique brings about new chances and challenges for the development of the communication techniques.